Conventionally, a technology for enhancing reception performance by carrying out two-dimensional spreading in an OFDM (Orthogonal Frequency Division Multiplexing)/CDMA (Code Division Multiple Access) communication system is described, for example, in Unexamined Japanese Patent Publication No. 2000-332724. The technology described in this Unexamined Japanese Patent Publication No. 2000-332724 arranges spreading chips not only in the time axis direction but also in the frequency axis direction in an OFDM/CDMA communication system so as to reduce inter-code interference caused by loss of orthogonality between the spreading codes.
However, the technology described in Unexamined Japanese Patent Publication No. 2000-332724 is intended to prevent loss of orthogonality between spreading codes in the OFDM/CDMA communication system, and therefore there is a problem that this technology is not capable to use for multicarrier communications other than CDMA scheme communications that do not use spreading chips. Furthermore, since an influence of frequency selective fading per spreading chip becomes a problem, it is not clear whether or not the technology described in Unexamined Japanese Patent Publication No. 2000-332724 is capable to obtain the same effect as a spreading chip even when symbols much longer than spreading chips are arranged two-dimensionally. Moreover, normally in a multicarrier communication, multicarrier signal is subjected to error correcting coding processing such as turbo coding and convolutional coding, and therefore when symbols are arranged two-dimensionally, it is necessary to consider their arrangement in units of code blocks generated through the error correcting coding processing. For this reason, when code blocks are arranged two-dimensionally, it is necessary to consider not only the influence of frequency selective fading but also the influence of multipaths and fading.
Generally, the error rate characteristic of an error correcting code such as turbo code and convolutional code is such that the error rate decreases as the variation of reception quality (e.g., likelihood per bit) of code blocks generated through the error correcting coding processing decreases, while the error rate increases as the variation of quality increases (see FIG. 1A to FIG. 1D).
Furthermore, the likelihood per bit depends on the quality per symbol after modulation, that is to say, SNR (Signal to Noise Ratio) and suchlike. For example, when data having 100 bits is subjected to error correcting (FEC) coding at a coding rate R=1/2 and transmitted in QPSK symbols, a signal having 200 bits are generated through FEC coding processing and QPSK symbols are transmitted in 2 bits per one symbol, and therefore 100 QPSK symbols are transmitted. The transmitted QPSK symbols are received by a receiver through a propagation path, but at this time, when an SNR changes for every QPSK symbol, the likelihood changes for every 2 bits after decoding. Deterioration of FEC performance due to the above-descried variation in data quality causes a problem that the error rate characteristic of a signal after error correcting deteriorates when a variation in SNR per symbol in a code block is large, even if average reception quality of a received signal, for example, SNR, is the same,
The deterioration of the error rate characteristic due to such SNR variation per symbol in a code block results in a serious problem in a mobile communication system using OFDM signals. A mobile communication system using OFDM signals is affected by the SNR variation in the time axis direction due to fading and affected by the SNR variation in the frequency axis direction due to frequency selective fading caused by multipaths. At this time, there is a feature that the variation in the time axis direction increases as the moving speed of the receiver increases, while the variation in the frequency axis direction increases as a maximum delay time of multipath signals between the transmitter and receiver increases. Furthermore, interference from other cells also increases a great deal for each subcarrier or for each symbol of an OFDM signal. For this reason, especially in cell edge, an SNR per symbol in 1 frame of the OFDM signal fluctuates a great deal, causing reception performance of the OFDM signal to deteriorate.